The present invention relates to a semiconductor memory device and, more particularly, to an MIS (Metal-Insulator-Semiconductor) dynamic memory device of a one-transistor and one-capacitor type.
In general, an MOS (broadly, MIS) dynamic memory device of a one-transistor and one-capacitor type incorporates memory cells, each comprising a transistor and a capacitor. In this case, the transistor serves as a switching gate for charging or discharging the capacitor. In addition, the presence of charges in the capacitor corresponds to the data "0" or "1". For example, the state in which the capacitor is discharged corresponds to the data "0", while the state in which the capacitor is charged corresponds to the data "1".
The above-mentioned transistor has a gate connected to a word line, a drain connected to a bit line and a source. The source is connected to one electrode of a capacitor. This capacitor has another electrode connected to a power supply, which is usually the ground or another power supply V.sub.cc. In the stand-by state, the bit line is precharged by the power supply V.sub.cc.
Therefore, if the data "0" is stored in the memory cell, a charge Q.sub.1 stored in the capacitor thereof is EQU Q.sub.1 =0.
Contrary to the above, if the data "1" is stored in the memory cell, a charge Q.sub.2 stored in the capacitor thereof is EQU Q.sub.2 =C(V.sub.cc -V.sub.th)
where C is the capacitance of the capacitor;
V.sub.cc is also a voltage of the power supply V.sub.cc ; and PA1 V.sub.th is a threshold voltage of the transistor. Therefore, the difference .DELTA.Q between the charges Q.sub.1 and Q.sub.2 is EQU .DELTA.Q=C(V.sub.cc -V.sub.th)
However, in the above-mentioned memory cell in which the other electrode of the capacitor is connected to the power supply V.sub.cc or the ground, in other words, the potential of the other electrode is definite, the difference .DELTA.Q is relatively small. This leads to an unstable read operation, since the operation of a sense amplifier for amplifying the difference in potential between the bit lines is dependent upon the difference .DELTA.Q.